1. Field of the Invention
This invention relates to the field of steel alloys, more particularly chromium alloys, which are noted for their hardness and their ability to withstand abrasion and repeated impact shock loads. More specifically this invention relates to the manufacture of cast steel alloys particularly well suited for fabrication into elements which will be subjected to abrasion and repeated impacts. Accordingly, the general objects of the present invention are to provide novel and improved materials and methods of such character.
2. Description of the Prior Art
Grinding members of various compositions and characteristics are widely employed in industrial processes which involve the fragmentation of materials. These prior art grinding members are manufactured out of forged steel or are cast from pearlitic white iron, martesitic white iron or chromium-alloyed steels.
For most applications cast grinding members made of chromium-alloyed steels are more resistant to abrasion than are grinding members made of ordinary forged steel or of pearlitic white cast iron. Such chromium-alloyed steels are also more resistant to repeated impacts than members made of martensitic white cast iron.
Hitherto known chromium-alloyed steels used for the manufacture of grinding media have, in addition to iron, had the following average composition by weight:
1.5% to 3% of C PA1 0.5% to 1.5% of Mn PA1 1% of Si (maximum) PA1 0.06% of S PA1 0.06% of P PA1 11% to 13% of Cr PA1 the chromium and carbon contents being comprised within the range defined by a closed area of a chromium-carbon coordinate diagram having the following coordinates: PA1 the molybdenum content being not more than 2% PA1 the content of the elements vanadium, tungsten, niobium, titanium and tantalum being not more than 1% PA1 and the balance being substantially iron with the percentages of manganese, silicon, phosphorus and sulphur not exceeding those normally contained in commercial cast steel or iron.
Additionally, these known steel alloys may contain up to 0.8% molybdenum and/or up to 0.7% nickel. In the case of balls intended for use in grinding mills, and depending on the size of the members formed therefrom, the characteristics of these known chromium-alloyed steels are as follows:
______________________________________ .phi. equal to or greater .phi. less than 50 mm than 50mm ______________________________________ Rockwell C Hardness 50 to 66 51 to 63 Residual austenite 5% to 13% or 5% to 13% or content more more ______________________________________ (where .phi. is the diameter of the balls tested).
As can be noted from the above table, the residual austenite content in the matrix of these known chromium-alloyed steels always exceeds 5% and increases with hardness at least in the range of relatively low austenite contents. In the table, the lower contents of residual austenite do always relate to the lower hardness, whereas with the increase of the hardness the residual austenite content tends toward the higher amounts indicated. However, as is known by those skilled in the art to which this invention pertains, while the resistance to abrasion of an alloy is enhanced as hardness increases, an increase in the residual austenite content in the matrix of the alloy results in a decrease in resistance to repeated impacts.
Grinding members manufactured from such hitherto known chromium-alloyed cast steels; although they are superior to the other above mentioned known grinding members, especially as regards their resistance to abrasion if these steels have been manufactured with a hardness in excess of 58 Rockwell C; generaly do not possess sufficient resistance to repeated impacts. This is particularly true for grinding members having diameters equal to or exceeding 50 mm. Such "large" diameter members are at present usually subjected to severe impact conditions. The lack of impact resistance results from the fact that for cast steel with Rockwell C hardness exceeding 58 the residual austenite content within the matrix remains too high (7% and over). Accordingly, under the action of impacts internal stresses are generated which can cause a premature destruction of the grinding members by breakage or spalling.